The invention relates to exposure drums for use in computerized, laser-direct-draw apparatus for making printing plates for newspaper printing, and more particularly to exposure drums used for making plates for printing newspapers in collect-run systems.
FIG. 7 illustrates a conventional newspaper rotary printing press in which webs of paper 2, fed from supply units S, are printed on both sides in three printing units 1, transferred in overlapped relationship, i.e. one over the other, into a folding unit T where the webs are cut and folded, and discharged as newspapers via a delivery fun U to a transfer apparatus (not shown). Also shown in FIG. 7 are printing cylinders 4 and blanket cylinders 5.
When the printing press shown in FIG. 7 is used in, for example, a single-imposition mode, a pair of printing plates H.sub.1 and H.sub.2 having four printing faces A, B, V, and X for four pages of a newspaper (see FIG. 12), and four printing faces C, D, Y, and Z (for four pages) respectively are mounted on the printing cylinder 4 of each printing unit 1 as shown in FIG. 8. Accordingly, each printing unit 1 can print 16 pages (eight pages on either side of a sheet). That is, 48 pages of a newspaper (16 pages.times.3 units) can be printed by the printing press shown in FIG. 7.
Similarly, when the printing press shown in FIG. 7 is used in a double-imposition mode, a pair of printing plates H.sub.3 and H.sub.4 having four printing faces A, B, A, and B for four pages of newspaper, (see FIG. 13), and four printing faces C, D, C and D (for four pages) respectively are mounted on the printing cylinder 4 of each printing unit 1, as shown in FIG. 9. Accordingly, each printing unit 1 can print 8 pages (four pages on either side of a sheet). Totally, 24 pages (8 pages.times.3 units) can be printed by the printing press. The number of the printing units 1 employed may depend on the number of pages of a finished newspaper.
The printing plates H.sub.1, H.sub.2, H.sub.3 and H.sub.4 to be mounted on the printing cylinders 4 are produced by a known computer-to-plate (CTP) method which includes the steps of mounting a photosensitive printing-plate material F (which may be an OPC sheet having an organic photoconductive layer deposited on an aluminum substrate or a PS sheet having a photosensitive coating applied on a thin base plate) on a cylindrical surface of an exposure drum E, rotating the exposure drum E while laterally moving a laser scanning head (an optical carriage) I, and drawing an image J directly on the photosensitive printing-plate material F by controlling the laser beam with an image signal from a computer G, as shown in FIG. 10. Referring to FIG. 10, there are provided a motor M.sub.2 for driving the optical carriage I in a direction denoted by an arrow, a laser-beam source 6, a multi-beam generation element 7, an acoustic optical modulator 8, an exposure device 9, and a lens 10, the arrangement of which is a known, so-called, multi-beam optical system (see Japanese Patent Application No. 8-293494).
The exposure drum E has a width W' of about 790 mm (equal to the total of a horizontal measurement L.sub.4 of two pages of a newspaper+.alpha..sub.4 =about 690 mm+100 mm) and a circumference length L' of about 1250 mm (equal to the total of a vertical measurement L.sub.2 of two pages of the newspaper+.alpha.=about 1160 mm+90 mm), as shown in FIGS. 11, 12 and 13. The exposure drum E is used to produce the four-face plates H.sub.1 and H.sub.3 (shown in each of FIGS. 12 and 13), four-face plates H.sub.2 and H.sub.4, full-spread type four-face plates H.sub.1a and H.sub.3a (shown in FIGS. 14 and 15 respectively), paired-vertical two-face plates H.sub.1b and H.sub.3b (shown in FIGS. 16 and 17 respectively), and single-vertical two-face plates H.sub.1c and H.sub.3c (shown in FIGS. 18 and 19 respectively).
Also illustrated in FIG. 11 are several (three or four) engaging pins 13' for holding the plate material F. Furthermore, ten to fifteen suction apertures 14' are provided in the exposure drum E for holding an end of the plate material F by suction.
It is noted that one page of an ordinary daily newspaper has a horizontal measurement L.sub.3 (FIGS. 16-19) of about 345 mm (about 13.5 inches) and a vertical measurement L.sub.1 of about 580 mm (about 22.8 inches).
In recent years, as advertisements on newspapers are diversified and newspaper article contents are varied from district to district, a variety of methods of printing newspapers have been correspondingly developed. One of these methods is a collect-run method illustrated in FIG. 20 in which a sheet P (having 4 pages including pages A and B) and another sheet Q (having 4 pages including pages V and X) are overlapped onto one another and folded in two. Many other sheets also folded in two are inserted into the folded sheets such that central fold lines of all the sheets folded in two are arranged laterally. Then, all the sheets are finally folded together again into a quatro form.
It is common in the collect-run method to replace one page of article contents (or page contents) with another. For this replacement, a whole four-face plate H.sub.1, H.sub.2 or H.sub.3, H.sub.4 on the plate cylinder 4 has been exchanged with a new one.
Even if only one page, for example, of V or A face, has needed to be replaced, the whole four-face plate H.sub.1 or H.sub.3 (FIGS. 12 or 13) has been replaced. A replacement of the bulky four-face plate H.sub.1 or H.sub.3 is not only time-consuming, but is also costly and uneconomical because a plate is wasted.
The same drawback exists when replacing the paired-vertical two-face plates H.sub.1b or H.sub.3b shown in FIGS. 16 or 17, and the single-vertical two-face plate H.sub.1c or H.sub.3c shown in FIGS. 18 or 19. Replacement of a whole two-face plate because of a necessity to replace only one page is also uneconomical.
It is an object of this invention to provide a novel exposure drum for an apparatus for making printing plates for newspaper printing capable of making such newspaper printing plates at high efficiency using the CTP method wherein any page of a newspaper is easily replaced with another desired page, with a loss of plate materials which results from such replacement when using four-face plates H.sub.1, H.sub.2, H.sub.3 and H.sub.4, and paired-vertical two-face plates H.sub.1b or H.sub.2b (not shown), H.sub.3b and H.sub.4b (not shown), being minimized.